Oxytocin is a peptide hormone which stimulates contraction of the uterine muscles, and it is believed to be involved in the etiology of pre-term labor and dysmenorrhea. Oxytocin antagonists have proved to be useful in the control of these conditions, and oxytocin antagonist peptides of good potency and selectivity for therapeutic use are disclosed in WO 95/02609, published 26 Jan. 1995. They are often intended for administration in aqueous solution, and the manufacture of ready-for-use doses of such antagonists may require that such solutions be stable for extended periods; which they may not always be. The potential need to prepare such a medicament immediately prior to use was considered to be inconvenient and generated an improvement.
U.S. Pat. No. 6,143,722 (EP 938,496; WO 98/23636) discloses equivalent heptapeptide analogues that exhibit oxytocin antagonist activity, which resemble those disclosed in the earlier WO 95 application, but wherein the C-terminus of the peptide is reduced to an alcohol. By heptapeptide or equivalent heptapeptide, for purposes of this application, is meant a cyclic compound where the N-terminal residue is deaminated and its side chain is linked by a covalent bond to a side chain of a residue spaced apart therefrom in the peptide chain which contains 6 residues in addition to the N-terminal residue.
Although such oxytocin antagonist peptides can be synthesized by the synthesis disclosed in the '722 patent, it requires about 7 separate steps, counting the peptide synthesis as one and not counting the synthesis of the modified homocysteine (Hcy) residue. More economical syntheses are frequently sought for chemical compounds of potential commercial interest, and such is the case in this instance.